1. Field of the Invention
The present invention relates to solar energy apparatus, in which solar radiation is collected and converted to another form of energy.
2. Description of the Prior Art
The increasing awareness and concern over energy use and conservation has led to increasing use of solar energy apparatus which converts solar radiation to another form of energy. One widely used type of solar energy apparatus converts solar radiation to thermal energy.
One solar energy apparatus of this general type has reflectors or lenses which collect, concentrate, and direct solar radiation onto an absorber containing a heat transfer medium. The collecting and concentrating optics, as well as the absorber, are located in the outside environment, such as on the roof of a building. This type of prior art solar energy apparatus has several disadvantages.
First, the collector apparatus in a typical system is 60% or less efficient due to reflection losses at primary and secondary reflectors, absorption losses through transparent absorption enclosures, heat losses to the environment, and inefficiencies of the heat transfer systems.
Second, the heat transfer medium is circulated through the outdoor environment. In severe climates, this limits the selection of the heat transfer medium, and increases the initial cost.
Third, achievable temperatures are limited by the temperature range of the heat medium and by the principles of heat transfer mechanics. The use/storage apparatus cannot achieve temperatures any greater than the temperature of the heat transfer medium.
Fourth, energy is consumed in continuously circulating the heat transfer medium between the absorber and the use/storage apparatus.
U.S. Pat. No. 3,905,352 by Jahn shows a prior art solar energy apparatus which utilizes an array of movable flat reflectors to concentrate solar radiation on a solar boiler, absorber tube, or similar apparatus. This prior art apparatus requires absorber areas equal in size to the area of the individual reflector, regardless of field size or focal distance. In addition, it requires a complex apparatus for moving the various flat mirrors.
Prior art solar energy apparatus have also used aspheric reflectors rather than flat mirrors like those shown in the Jahn patent. These aspheric reflectors are capable of concentrating solar radiation at a focal line or focal point of the reflector. The focal line or focal point, however, must remain in the same position relative to the reflector.
Another type of prior art solar energy apparatus uses optical waveguides, typically in the form of fiber optic bundles, to collect and transmit solar radiation. U.S. Pat. Nos. 3,467,840 by Weiner; 3,780,722 by Swet; 4,026,267 by Coleman; and 4,029,519 by Schertz et al describe solar energy apparatus using optical waveguides.
One difficulty of the prior art solar energy apparatus using optical waveguides is that the optical waveguides typically accept radiation only from a relatively small angle of incidence. Since the sun's position in the sky changes throughout the day, this significantly reduces the efficiency of the system. For example, in U.S. Pat. No. 4,026,267 by Coleman, fixed lenses are used for directing solar radiation onto the ends of fiber optic bundles. As shown, a significant amount of solar radiation will only be directed onto the ends of the fiber optic bundles when the sun's position is perpendicular to the plane of the array of lenses, which occurs only during a relatively short period of any day.